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United States Patent |
6,027,221
|
Ishikawa
,   et al.
|
February 22, 2000
|
Surface light source device of side light type and mold for light guide
plate employed in the device
Abstract
In a surface light source device of side light type applied to a liquid
crystal display or the like, a light control member is prevented from
clinging to a light guide plate. The light guide plate is formed by means
of injection molding with molds 16A and 16B. The molds 16A and 16B have an
additional cavity 13 in a portion corresponding to a position close to an
incidence surface of the light guide plate 11, with a gate G being
arranged in the cavity. In the mold 16B, its inside surface corresponding
to an emitting surface 12 is roughened. According to gradient in
roughness, the emitting surface of a light guide plate to be manufactured
is roughened so as to have uniform or graded roughness. On a center line
C--C, the inside surface is much roughened in a portion distant from the
cavity 13. Further, on a crossing line D--D, the inside surface is
roughened so as to reduce the roughness according to distance from the
center. If the gradient in roughness on the inside surface of the mold 16B
is intensified, the emitting surface of the light guide plate may be
roughened so as to have graded roughness. It is thereby possible to
prevent clinging of the control member from occurring even when
deformation of the light guide plate occurs. (FIG. 5b)
Inventors:
|
Ishikawa; Tsuyoshi (Tokyo, JP);
Yamazaki; Hiroshi (Higashimatsuyama, JP)
|
Assignee:
|
Enplas Corporation (Kawaguchi, JP);
Yasuhiro Koike (Yokohama, JP)
|
Appl. No.:
|
904348 |
Filed:
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July 31, 1997 |
Foreign Application Priority Data
Current U.S. Class: |
362/619; 362/330; 362/331 |
Intern'l Class: |
F21V 007/04 |
Field of Search: |
362/31,26,268,330,331,332,339
|
References Cited
U.S. Patent Documents
5126882 | Jun., 1992 | Oe et al. | 359/619.
|
5833344 | Nov., 1998 | Arai et al. | 362/31.
|
5844720 | Dec., 1998 | Ohara et al. | 362/339.
|
Primary Examiner: Day; Michael H.
Assistant Examiner: Hopper; Todd Reed
Attorney, Agent or Firm: Staas & Halsey LLP
Claims
What is claimed is:
1. A surface light source device of side light type comprising:
a light guide plate with emitting directivity;
a light source for supplying illumination light through a side end surface
of the light guide plate which has an emitting surface to emit said
illumination light; and
a light control member arranged along said emitting surface to modify
directivity of the illumination light emitted from said emitting surface,
wherein said emitting surface is provided with roughness to prevent said
light control member from clinging to said emitting surface, without
losing emitting directivity of said light guide plate,
said roughness being intensified in a portion distant from said side end
surface.
2. A surface light source device of side light type according to claim 1,
wherein said light guide plate is shaped so as to have thickness which is
reduced according to distance from said side end surface.
3. A mold used for forming a light guide plate employed in a surface light
source device of side light type, which includes a light guide plate with
emitting directivity, a light source for supplying illumination light
through a side end surface of the light guide plate which has an emitting
surface to emit said illumination light, and a light control member
arranged along said emitting surface to modify directivity of illumination
light emitted from said emitting surface, said emitting surface being
roughened without losing emitting directivity of said light guide plate to
prevent said light control member from clinging to said emitting surface,
wherein an inside surface of said mold is roughened in a portion
corresponding to said emitting surface of said light guide plate; and
the roughness given to the inside surface of said mold is intensified in an
area distant from another area corresponding to the vicinity of the side
end surface of said light guide plate, in comparison with the area
corresponding to the vicinity of said side end surface.
4. A mold according to claim 3, wherein the roughness given to the inside
surface of said mold is intensified in an area distant from another area
corresponding to the vicinity of the side end surface of said light guide
plate, in comparison with the area corresponding to the vicinity of said
side end surface, so as to give uniform roughness to said emitting surface
of said light guide plate.
5. A mold according to claim 3, wherein the roughness given to the inside
surface of said mold is intensified in an area distant from another area
corresponding to the vicinity of the side end surface of said light guide
plate, in comparison with the area corresponding to the vicinity of said
side end surface, so as to give intensified roughness to said emitting
surface of said light guide plate in a portion distant from the side end
surface supplied with said illumination light.
Description
BACKGROUND
1. Field of Invention
This invention relates to a surface light source device of side light type
applied to a liquid crystal display or the like, and a mold for a light
guide plate employed in the device, and more particularly, to a surface
light source device of side light type employing a light guide plate with
emitting directivity and a mold for forming the light guide plate.
2. Related Art
A surface light source device of side light type has been conventionally
applied to a liquid crystal display, for instance, and illuminates a
liquid crystal panel from the back surface. This arrangement is suitable
to reduce thickness of the device as a whole.
In the surface light source device of side light type, a rod-shaped light
source such as a cold cathode tube is usually employed as a primary light
source to be arranged besides a light guide plate (a plate-shaped light
guide). Illumination light emitted from the primary light source is
introduced into the light guide plate through a side end surface of the
light guide plate. The introduced illumination light propagates through
the light guide plate, and in this process, emission of light from a major
surface of the light guide plate occurs toward a liquid crystal panel.
A well-known light guide plate employed in the surface light source device
of side light type described above includes a light guide plate of a type
having an approximately uniform thickness and a light guide plate of a
type showing a tendency to reduce thickness according to distance from the
primary light source. Generally, light guide plates of the latter type
emit illumination light more efficiently than the former type.
FIG. 10 is an exploded perspective view showing a surface light source
device of side light type employing light guide plate of the latter type.
Referring to FIG. 10, a surface light source device of side light type 1
has a light scattering guide plate (a light guide plate made of a light
scattering guide) 2, a primary light source 3 arranged on the lateral side
of the light scattering guide plate, a reflection sheet 4 and a prism
sheet 5 functioning as a light control member. The reflection sheet 4, the
light scattering guide plate 2 and the prism sheet 5 are laminatedly
arranged.
The primary light source 3 has a cold cathode tube (a fluorescent lamp) 6
and a reflection member (a reflector) 7 of an generally semi-circular
section arranged around the cold cathode tube. Illumination light impinges
on a side end surface of the light scattering guide plate 2 through an
opening of the reflector 7. The reflection sheet 4 adopts a sheet-shaped
regular reflection member made of metal foil or the like, or a
sheet-shaped diffuse reflection member made of a white PET film or the
like.
The light scattering guide plate 2 has a wedge-shaped section, and consists
of a matrix made of polymethyl methacrylate (PMMA), for instance, and
light transmitting fine particles uniformly distributed in the matrix and
having deflective index different from that of the matrix.
FIG. 11 shows a section taken along a line A--A in FIG. 10. Referring to
FIG. 8, illumination light L from the primary light source 3 is introduced
into the light scattering guide plate 2 through an incidence surface T
provided by a side end surface of the light scattering guide plate 2, and
makes a propagation toward an end portion while undergoing repetitive
reflection between a plane (which will be hereinafter referred to as "a
slope"), along which the reflection sheet 4 is disposed, and a plane
(which will be hereinafter referred to as "an emitting surface"), along
which the prism sheet 5 is disposed. In the process, the illumination
light L undergoes scattering caused by the light transmitting fine
particles. If a reflection sheet 4 is made of a diffuse reflection member,
the illumination light is affected by an action of diffuse reflection as
well.
Every time the illumination light L repeats reflection on the slope, angle
of incidence with respect to the emitting surface is reduced little by
little. Reduction in angle of incidence results in an increase of a
component equal to the critical angle or less with respect to the emitting
surface, urging emission from the emitting surface onward. It is thereby
possible to prevent output light from being insufficient in an area
distant from the primary light source 3.
Illumination light L1 emitted from the emitting surface undergoes
scattering caused by the light transmitting fine particles or diffuse
reflection caused by the reflection sheet 4, and therefore, shows the
properties of scattered light. However, a main propagation direction of
the emitted illumination light L1 is tilted toward an end portion (a
direction reverse to the primary light source 3) with respect to the
emitting surface. That is, the emitted illumination light L1 has
directivity. The surface light source device of side light type 1
employing the above light guide plate generates illumination light having
emitting directivity.
The prism sheet 5 is made of a light transmitting sheet material such as
polycarbonate and has a prism surface. The prism sheet 5 is arranged so
that the prism surface faces the light scattering guide plate 2. The prism
surface is composed of a large number of projections, each of which has a
triangular section and runs substantially parallel to the incidence
surface T of the light scattering guide plate 2. The prism sheet 5
modifies a main emitting direction of the emitted illumination light L1
with a slope of each projection so as to emit the illumination light L1 in
a frontal direction of the emitting surface. There is also a case where a
prism sheet having prism surfaces on both surfaces is used. In this case,
projections forming one prism surface run in a direction respectively
orthogonal to projections forming the other prism surface.
In general, the surface light source device of side light type 1 employing
the light guide plate of the wedge-shaped section as described above emits
illumination light in a frontal direction more efficiently than surface
light source devices of side light type employing light guide plates
substantially uniform in thickness.
In some cases, surface light source devices of side light type may employ,
as a light guide plate with emitting directivity, a light guide plate
having a scattering film, a roughened surface or the like formed on one
surface and/or the other surface (i.e. on emitting surface and/or a back
surface) of a transparent member or a semi-transparent member in the shape
of a wedge or the like. Such light guide plates also emit illumination
light in a frontal direction efficiently.
OBJECT AND SUMMARY OF INVENTION
According to our observation, surface light source devices of side light
type as described above placed in a high-temperature environment for a
long time show various patterns on an outside surface (an emitting surface
of the surface light source device of side light type) of the prism sheet
5, as shown in FIG. 12.
As the result of making a study of factors leading to such various
patterns, it has been found out that the prism sheet 5 locally clings to
the emitting surface of the light guide plate 2 and an air layer between
the prism sheet 5 and the emitting surface is partially lost, resulting in
occurrence of such patterns.
When the prism sheet 5 clings to the emitting surface of the light guide
plate 2 to cover a certain extent, an insular-shaped pattern C occurs,
while when the prism sheet 5 clings in the shape of dots to the emitting
surface of the light guide plate 2, a dot-shaped pattern D occurs. Such
patterns remarkably degrades a quality in display, when the surface light
source device of side light type is applied to back lighting in a liquid
crystal display.
An object of the present invention is to prevent a light control member
from clinging to an emitting surface of a light guide plate employed in a
surface light source device of side light type. Another object of the
present invention is to enhance applicability of a surface light source
device of side light type to back lighting in a liquid crystal display or
the like by avoiding such a clinging phenomenon.
A further object of the present invention is to provide a mold suitably
used to form a light guide plate required to prevent the above clinging
phenomenon from occurring in a surface light source device of side light
type.
The present invention prevents a light control member from clinging to an
emitting surface by improving a surface light source device of side light
type comprising a light guide plate with emitting directivity, a light
source for supplying illumination light through a side end surface of the
light guide plate, and a light control member arranged along the light
guide plate so as to modify directivity of illumination light emitted from
an emitting surface. The light guide plate is preferably shaped to have
thickness which is reduced according to distance from a side end surface
subjected to supply of light.
According to a feature of the present invention, the emitting surface is
roughened without losing emitting directivity of the light guide plate for
the purpose of preventing clinging from occurring. Although it is possible
to uniformly roughen the emitting surface, the roughness of the emitting
surface is preferably intensified in a portion distant from the side end
surface supplied with illumination light for the purpose of enhancing a
function of preventing clinging from occurring in deformation.
The present invention also provides a mold for forming a light guide plate
employed in the surface light source device of side light type as
described above. An inside surface of the mold is roughened in a portion
corresponding to an emitting surface of the light guide plate. The
roughness of the inside surface is intensified in an area distant from
another area corresponding to the vicinity of a side end surface subjected
to supply of light in the light guide plate, in comparison with the area
corresponding to the vicinity of the side end surface described above. As
the result of adjusting the degree of intensification in roughness, it is
possible to roughen the emitting surface of the light guide plate provided
as a molded product so as to have uniform or graded roughness.
Hereinafter will be described the present invention in more detail with
reference to accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a perspective view showing a light guide plate (a light
scattering guide plate) employed in a surface light source device of side
light type according to the first embodiment of the present invention;
FIG. 2 is an enlarged-scale sectional view showing an emitting surface of
the light guide plate shown in FIG. 1;
FIG. 3 is a graph for explaining influence of roughening in a light guide
plate upon directivity;
FIG. 4 shows a characteristic curve of luminance in a direction
perpendicular to an emitting surface, with roughness being set as a
parameter;
FIG. 5a is a side view for explaining a mold used for manufacturing the
light guide plate shown in FIG. 1;
FIG. 5b is a plan view for explaining a mold used for manufacturing the
light guide plate shown in FIG. 1, together with a graph for explaining a
distribution of roughness on an inside surface of the mold;
FIG. 6 is a plan view showing a flow of a resin in case of forming the
light guide plate shown in FIG. 1;
FIG. 7 is a plan view showing a flow of a resin in forming, subsequently to
the flow shown in FIG. 6;
FIG. 8 is a plan view for explaining a case where clinging of a prism sheet
occurs when an inside surface of a mold is uniformly roughened;
FIG. 9 is a side view showing deformation occurring in a light guide plate
when residual stress is relieved;
FIG. 10 is an exploded perspective view showing a surface light source
device of side light type in a prior art;
FIG. 11 is a sectional view taken along a line A--A in FIG. 10; and
FIG. 12 is a front view for explaining clinging of a prism sheet to an
emitting surface in a surface light source device of side light type in a
prior art.
PREFERRED EMBODIMENTS
In the following description, the reference numerals used in FIGS. 10, 11
and 12 are accordingly used in common to avoid repeated descriptions.
(1) First Embodiment
FIG. 1 is a perspective view showing a light guide plate, which is employed
in a surface light source device of side light type according to the first
embodiment of the present invention. The surface light source device of
side light type in the present invention is identical with the surface
light source device of side light type described with reference to FIGS.
10 and 11, except that an emitting surface 12 of a light guide plate 11
with emitting directivity is roughened.
The light guide plate 11 is made of a light scattering guide, and the
emitting surface 12 of the light guide plate is matted so as to form a
matted surface (a roughened surface) having uniform roughness such that
arithmetic mean roughness Ra is in the range of 0.02 to 0.25 .mu.m.
Arithmetic mean roughness Ra denotes a unit of surface roughness defined by
JIS B0031-1994. The roughening in the emitting surface effectively
prevents a prism sheet 5 functioning as a light control member from
clinging to the emitting surface 12.
A description will now be given of the reasons with reference to FIG. 2
showing an enlarged section of the prism sheet 5 and that of the emitting
surface 12 respectively employed in this embodiment. The prism sheet 5 is
a molded product made of polycarbonate and has a large number of
projections, each of which has a triangular section. In each projection of
this embodiment, a height H1 is in the range of 20 to 40 .mu.m, and each
pitch W1 is 50 .mu.m.
The emitting surface 12 is roughened through the above matting treatment,
and as a result, a large number of irregular portions are formed at random
on the emitting surface 12. In each irregular portion, a depth H is in the
range of 0.05 to 0.2 .mu.m, and a mean pitch W is in the range of 5 to 40
.mu.m. Since these irregular portions hold the prism sheet 5 at their tip
portions, adhesiveness between the prism sheet 5 and the emitting surface
12 is reduced, thereby effectively preventing the prism sheet 5 from
changing.
However, the roughened emitting surface 12 generates scattered output
light, and as a result, reduces directivity of output light. This fact is
illustrated in a graph of FIG. 3 showing directivity in a case of
employing the light guide plate shown in FIG. 1. In this graph, the
transverse axis represents emitting angle of illumination light emitted
through the prism sheet 5, and the ordinate axis represents relative
luminance. The direction perpendicular to the emitting surface 12 is
defined as an angle of 0.degree., and directions approaching an end in a
wedge shape is defined as positive direction. Curves respectively denoted
by reference numerals M1, M2 and M3 correspond to three roughness
conditions concerning the emitting surface of the light guide plate.
Curve M1: specular surface
Curve M2: roughened surface of 0.2 .mu.m in arithmetic mean roughness Ra
Curve M3: roughened surface of 0.3 .mu.m in arithmetic mean roughness Ra
In comparison of these curves with one another, there is no great
difference in directivity between the curve M1 (specular surface) and the
curve M2 (Ra=0.2 .mu.m), while a remarkable deduction in directivity
occurs in the curve M3 (Ra=0.3 .mu.m). It is read from a comparison
between the curve M3 and the curve M1 that luminance (quantity of light)
in a frontal direction is reduced about 10%.
A graph of FIG. 4 will give an understanding of an influence of roughening
in the light guide plate upon directivity in more detail. The graph of
FIG. 4 shows the result of measurement in luminance (the ordinate axis) in
a direction perpendicular to the emitting surface on condition that the
roughness (arithmetic mean roughness Ra) of the emitting surface of the
light guide plate is set as a parameter (the transverse axis).
As is read from this graph, when the arithmetic mean roughness Ra reaches
0.25 .mu.m or above, directivity is sharply deteriorated, and a luminance
level in a frontal direction is reduced. Such a reduction in luminance
level is not a preferable matter.
On the other hand, for the purpose of verifying a performance of preventing
clinging of the prism sheet 5 from occurring by means of roughening the
emitting surface 12 of the light guide plate 11, the prism sheets 5 were
respectively laminated on a light guide plate 11 having an emitting
surface 12 roughened so as to have various degrees of roughness, and the
resultant was allowed to stand for 1000 hours in an environment at the
temperature of 70.degree. and the humidity of 20%. As the result of making
observations of clinging of the prism sheet 5 after the resultant has been
allowed to stand for 1000 hours, it came out that clinging easily occurs
when Ra is approximately 0.02 .mu.m or less, while clinging hardly occurs
when Ra is approximately 0.02 .mu.m or above.
From the above fact, the emitting surface 12 of the light guide plate 11
should be roughened in order to avoid deterioration of directivity and
clinging of the prism sheet 5, preferably on condition that the arithmetic
mean roughness Ra is within the range of 0.02 to 0.25 .mu.m.
FIGS. 5a and 5b are a side view and a plan view respectively showing a mold
used for forming the light scattering guide plate 11 by means of injection
molding. Molds 16A and 16B have an additional cavity 13 in a portion
corresponding to a position close to an incidence surface of the light
guide plate 11. The cavity 13 is provided with a gate G. In the molds 16A
and 16B, it is thereby possible to surely form the light guide plate 11 by
allowing a resin to flow from a portion corresponding to a thick end
portion (an incidence surface) of the wedge-shaped light guide plate
toward a portion corresponding to a thin end portion (an end portion) of
the wedge-shaped light guide plate.
In the mold 16B out of the molds 16A and 16B, an inside surface
corresponding to the emitting surface 12 is roughened. Irregularities of
the roughened surface of this mold are transferred to a molded product
(the light guide plate 11), and as a result, the roughened emitting
surface 12 is formed.
Further, in the mold 16B, the inside surface is roughened as described
above for forming the emitting surface 12 so that the roughness is graded.
That is, as shown in a graph, the inside surface is roughened so as to
intensify the roughness according as distance from the incidence surface
is increased, in a center line C--C of the light guide plate 11 orthogonal
to the incidence surface.
Further, the inside surface is roughened so as to reduce the roughness in
the vicinity of the end of the light guide plate 11 according as distance
from the center is increased, in a crossing line of D--D parallel to the
incidence surface.
Such a gradient in roughness as described above is required for
approximately uniformly roughening the emitting surface 12 in actuality
under the molding conditions suitable for manufacturing the light guide
plate 11 of this kind. The reasons why such a gradient in roughness is
required are as follows.
As shown in FIG. 6, when a thin plate-shaped product having a wedge-shaped
section like the light guide plate 11 is molded by forming the gate G on
one end surface, a resin having flown into the cavity preferentially flows
toward an easily flowing direction. In this case, the resin flows in a
direction along the incidence surface, and thereafter flows toward the end
in the wedge shape as shown by an arrow E.
At this time, the resin at both end portions flows ahead of that at a
center portion, and lastly flows in a center portion at the end in the
wedge shape, as shown in FIG. 7.
The advantage of introduction of the resin through a single gate
corresponding to the position of the incidence surface is that a weld line
is prevented from occurring, in comparison with a case where the gate is
provided in a plurality of portions. However, as described above, in a
portion, into which the resin lastly flows, fluidity of the resin is
reduced more than that in other portions, and as a result, capability of
transferring the irregularities formed on the inside surface of the mold
is reduced.
That is, if the light guide plate were manufactured by means of injection
molding without giving any gradient in roughness to the inside surface of
the mold, clinging of the prism sheet would easily occur in a portion
corresponding to an area (see reference character AR) where fluidity of
the resin is reduced, as shown in FIG. 8. In the present embodiment, the
gradient in roughness is given to the inside surface of the mold 16b as
described above for the purpose of avoiding the above drawback.
In this manner, according to this embodiment, it is possible to effectively
prevent the prism sheet 5 from clinging to the light guide plate 11, and
also to avoid the quality of output light from being deteriorated by
clinging of the prism sheet.
(2) Second Embodiment
In the second embodiment, the gradient in roughness on the inside surface
of the mold is intensified more than that in the first embodiment. It is
thereby possible to give a gradient in roughness also to the emitting
surface 12 of the light guide plate 11 within the range of roughness
described above. The gradient in roughness is given to the emitting
surface 12 so as to intensify the roughness toward the end in the wedge
shape.
The gradient in roughness on the emitting surface 12 is advantageous in
solving a problem resulting from corrugate deformation having a tendency
to occur in the vicinity of the end in the wedge shape. That is, in an
area (see reference character AR in FIG. 8) where fluidity of the resin is
reduced, residual stress easily occurs inside the molded product (the
light guide plate 11). When this residual stress is relieved due to
factors such as a great change in temperature in the course of a molding
process, corrugate deformation easily occurs in the vicinity of the end.
FIG. 9 is a side view showing deformation occurring in the light guide
plate when residual stress is relieved. It should be noted that the size
of deformation in FIG. 9 is exaggerated.
When such deformation occurs, press force between a protuberant portion and
the prism sheet 5 becomes different from that between a recess portion and
the prism sheet 5. In the protuberant portion, clinging of the prism sheet
5 very easily occurs.
In this embodiment, the emitting surface 12 of the light guide plate 11 is
roughened so as to intensify the roughness in the vicinity of the end in
the wedge shape, and as a result, it is possible to prevent the prism
sheet 5 from clinging to the emitting surface 12 even if the deformation
as described above occurs. Further, it is possible to prepare a mold, in
which roughness on the inside surface in an area (corresponding to a
portion of the emitting surface in the vicinity of the end of the
wedge-shaped light guide plate) distant from the cavity 13 is intensified
more than that in the first embodiment, for forming such the light guide
plate.
(3) Further Embodiment
In the above embodiments, while the prism sheet having a single prism
surface is employed as a light control member, the present invention is
not limited to the above embodiments. As a matter of course, the present
invention may be also applied to a surface light source device of side
light type employing a prism sheet (a so-called double prism sheet) having
both surfaces formed into prism surfaces.
Further, the light scattering guide plate having the wedge-shaped section
is employed as a light guide plate. However, the present invention is not
limited to the above light scattering guide plate. In addition to a light
guide plate generally showing a tendency to reduce thickness according as
the light scattering guide plate becomes more distant from a light source,
it may be possible to employ a plate-shaped light scattering guide plate.
The light guide plate may be made of a transparent light guiding material,
instead of the light scattering guide.
Further, in the above embodiments, discussion has been given on a case
where illumination light is introduced through one side end surface, the
present invention is not limited to the above embodiments, and may be
applied also to a surface light source device of side light type having a
structure of introducing illumination light through one side end surface
together with the other side end surface.
Further, in the above embodiments, a description has been given of a case
where the present invention is applied to the surface light source device
for a liquid crystal display, the present invention is not limited to the
above embodiments. The present invention may be widely applied to a
surface light source device of side light type for various illumination
apparatuses and displays.
As described above in detail, the present invention prevents a light
control member employed in a surface light source device from clinging to
an emission surface and also prevents uniformity in illumination from
decreasing which would be caused by the clinging. Further, the present
invention provides a mold for forming the light guide plate employed in
the surface light source device of side light type described above.
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